High voltage suspension insulators are used to suspend power transmission lines from overhead supports on poles and towers. Older suspension insulators are made of strings of porcelain insulators having a size and shape required of that material to provide the necessary mechanical strength, dielectric strength and creepage distance. To provide the necessary mechanical and electrical characteristics, porcelain insulators are heavy. Moreover, such porcelain insulators are expensive to install, and require stronger supporting structures. Additionally, porcelain is brittle and subject to damage during shipment and installation.
Newer insulators are formed of a fiberglass reinforced polymer rod and an external protective housing forming the weathersheds. The weathershed housing is usually made of an elastomer or an epoxy material. Elastomer or epoxy weathershed housings are designed to protect the fiberglass reinforced rods from weather and electrical activity. Weather and electrical activity degrade the mechanical strength of the fiberglass reinforced rods. The weathersheds on the housings intercept water flow down the insulators and increase the distance along the surface of the insulator for better electrical performance in wet or contaminated conditions.
Metal end fittings are attached to the fiberglass rod either by epoxy or by crimping the metal fittings to the fiberglass rod, as disclosed for example in U.S. Pat. No. 3,898,372 to Kalb, the subject matter of which is hereby incorporated by reference. The mechanical strength of this insulator is dependent upon the strength of the fiberglass rod, the connection of the rod to the metal end fittings and the strength of the end fittings. Such strength is generally 15,000 pounds or greater.